Borehole treatment compositions encompass all kinds of auxiliary fluids which can be used when drilling into reserves of petroleum or natural gas. As a characteristic example of such treatment compositions, the invention is described below with reference to drilling fluids and drilling muds based thereon which may be used both with land-sited and with marine wells. The field of use of the inventive modification of auxiliary fluids of the type concerned here, however, is not restricted to these systems. It also embraces the numerous further auxiliary fluids of the field which is addressed here. Examples that may be mentioned include spotting fluids, spacers, packer fluids, auxiliary fluids for workover and stimulation and for fracturing. The invention is directed both to auxiliary fluids of the type stated which are oil-based, i.e. operate with a continuous oil phase, and to auxiliaries in which the oil phase is emulsified in a continuous phase which in particular is aqueous. Drilling fluids and drilling muds based on them are characteristic examples of the various possibilities.
On the one hand, are known water-based drilling muds which have an emulsified oil phase content of approximately 1 to 50%, in addition to the other customary auxiliaries of such a drilling mud, which are also referred to as O/W emulsion muds. On the other hand, oil-based mud systems in which the oil constitutes the flowable phase or else at least a substantial fraction of the flowable phase in the form of a continuous oil phase are in widespread use. Particular importance here attaches to the muds known as invert drilling muds, which on the basis of W/O emulsions comprise a disperse aqueous phase in the continuous oil phase. The amount of disperse aqueous phase is normally in the range from at least about 5 to 10% by weight up to about 50 to 60% by weight. In addition to these W/O invert drilling muds, those referred to as true oil muds are also known, whose liquid phase is composed almost exclusively of a continuous oil phase and which contain small amounts if any—normally not more than about 5 to 10% by weight—of aqueous phase in dispersed form.
The non-aqueous phase of such mud systems is formed by what is called the carrier fluid. This is generally diesel oil, to which certain additives are added to form the actual drilling mud. With the end of the 1980s, however, came the demand for environment-friendly mud systems and hence carrier fluids. These were in particular to be more biodegradable than the diesel oil used beforehand. Alongside various liquid esters, such as those described in EP 0 374 672 A1, for example, olefinic hydrocarbons were investigated for their usefulness. Reference may be made here, by way of example, to the applicant's EP 0 765 368 A1, which is directed to the use of alpha-olefins as carrier fluids.
There nevertheless continues to be a need for novel carrier fluids, since the known compounds frequently cannot be used under all operating conditions or do not meet the increasingly stringent requirements for biodegradability or low toxicity. The latter criterion in particular is becoming increasingly significant, since more and more wells are destined to be sunk in ecologically sensitive regions. This is especially the case with offshore wells, i.e. wells sunk in the ocean floor.
The toxicity of a mud is generally determined by biological tests in which marine microbiota are exposed to the ingredients of the mud at different concentrations. The objective is to find mud systems which meet the physical, technical requirements while being of minimal toxicity to the environment. Mention should also be made at this point of the physical stresses on the drilling muds. The muds are heated at great depths to high temperatures, up to 250° C. or more; high pressures prevail, and at the same time the compounds in the mud both must remain chemically stable and must not exhibit any severe change in their viscosity behaviour, while continuing to form a stable emulsion under the conditions outlined.
It is an object of the present invention to provide borehole treatment compositions, especially drilling muds, which are enhanced both in respect of their performance properties and in respect of environmental compatibility.